docs: add message batching for AgentCore Memory

[jariy17]

Summary

• Document the batch_size parameter added to AgentCoreMemoryConfig in bedrock-agentcore-sdk-python#244
• Add a new Message Batching section with context manager and explicit close() examples
• Update the Basic Setup example to use the recommended with statement pattern
• Add a best-practice note about flushing buffered messages to the Important Notes section

Test plan

• [x ] Run mkdocs serve locally and verify the AgentCore Memory page renders correctly
• [ x] Confirm the parameter table includes batch_size and renders properly
• [ x] Verify code examples are syntactically correct
• [ x] Check that admonitions (warning and note) display correctly

[strands-agent]

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[afarntrog]

Thanks @jariy17 !